High voltage and medium voltage transformers are widely used in electrical power distribution of today. Utilizing the magnetic features of electrical currents, they transfer power between two or more incompatible electrical AC-circuits. Thereby, power from a power plant can be transported by a small current of very high voltage and then stepped down to a large current of low voltage before reaching the users.
Supply authorities are under obligation to their customers to maintain the supply voltage between certain limits. A tap changer is a device used in a transformer for regulation of the transformer output voltage within these limits. Normally, this is achieved by changing the ratios of the transformers of the system by altering the number of turns in one winding of the appropriate transformer(s). A lower load on the system may for instance require that tap-changing operations decrease the number of turns in the winding. This ultimately results in an increased output voltage as compared to if no tap changing were performed. Normally, a tap changer performs in the range of 5–20 changing operations per day, but for very demanding systems, such as melting furnaces, there may be hundreds of such operations per day.
Besides the described application, tap changers may also be used in connection with other inductive power devices such as reactors. Tap changers are either on-load, i.e. operating while the transformer is energized, or off-load and there is a wide range of models available. A tap changer generally comprises a number of switches for tap changing and a number of resistors or other impedances to prevent short-circuiting. Furthermore, the tap changer typically is filled with an insulating liquid, such as oil, which besides insulation offers cooling of the device.
Problems during switching in tap changers are not usual, but can be very serious when they occur. Such problems are mostly related to a long switching time, in turn a result of e.g. wear or broken details. A major disadvantage is that the switching problems generally are not discovered before the tap changer becomes entirely unusable. Often the whole transformer has to be switched off in an emergency manner as a consequence of inadequate tap changing, which implies considerable inconveniences for the consumers as well as extremely high expenses for the power industry.
Inspection of a tap changer generally requires opening of the enclosure and removal of the switching mechanism from the oil. This causes a lengthy service interruption and is hence performed as seldom as possible. Thus, solving the error detection problem by tap changer inspections on a regular basis is not a desirable option.
It is known in the prior art to measure acoustic signals in tap changers to obtain information about the functional qualities thereof. According to U.S. Pat. No. 5,492,014 acoustic signals resulting from, among other things, tap-changing operations are measured. The “sound” of the operations is recorded and unusual sounds are used as indications of unusual events. Such acoustic measurements require specially adapted equipment and are often associated with problems due to resonance, spurious signals, disturbances, etc. Moreover, diagrams resulting from acoustic measurements are generally very difficult to interpret, in particular since every system has its own acoustic characteristics.
Accordingly, there is a strong demand for a method and means, which facilitates early detection of switching problems in a tap changer.